For books, you can spend a lot of money on Skennerton's volumes. They are as perfect as they can be made, although they might not be what you want straight-off, this strictly owning to the huge wealth of detail.
I would recommend three FREE books BEFORE (but not exclusive of) Skennerton's magna opi.
Zip over to milsurps dot com and take out a (free) membership. Once you have done that, you can start pillaging that wonderful Military Knowledge Library (MKL) they have set up.
FIRST book you download is your own copy of the wartime Manual: "Rifle, 1942". This is all the manual the average WW2 troopie ever would have seen.
Then you go one step up the food chain and get the Marksmanship Instructor's manual, "SHOOT TO LIVE!: THE JOHNSON METHOD OF MUSKETRY COACHING". This book is lavishly illustrated, Canadian, 1945 publication date and it is the BEST book ever written on marksmanship with iron-sighted military rifles. It was written with YOUR Number 4 as the demonstration rifle. You will NOT find a better book on shooting the LE Number 4.
Then you go for a HISTORY of the design and development of the entire LEE series of rifles, from the very first to the very last (almost), written while the last rifles were still being made and BEFORE the great majority of the original paper-work was destroyed. This is "THE LEE-ENFIELD RIFLE" by Major E.G.B. Reynolds. This is a rare book, out of print now for nearly half a century but it is still held in Copyright by the Publisher (and many of us are PRAYING that another printing might become available). By a special arrangement with the Publisher, Doug (BADGER) has received permission for Members of that Website ONLY to download reference .pdfs of the entire book, text and photos. Joining milsurps is the only way to obtain a LEGAL .pdf of the book. If you have an interest in the design, development and manufacture of these rifles, as well as in the (many) production problems, this is the ONLY book which discusses them intelligibly. I can NOT recommend this book highly enough. It ALSO has a most interesting chapter on the P-'13/P-'14 rifles designed at Enfield!
So that starts your library.
Going onward in the same place, likely you will find that the MKL is the single BEST source in the world for information on these rifles. The MKL has HUNDREDS of papers, documents, Armourer's Notes, videos, instructions....... you name it...... all on your rifle and its brothers and cousins. Th MKL is necessary for almost ALL collectors of military rifles; they have manuals on just about all of them and dozens of rare books to boot. And all free.
Hope this helps.